1. Field of the Invention
The present invention relates to an elastic wave filter device, and more particularly, to a 3-IDT longitudinally-coupled resonator-type elastic wave filter device including first to third IDT electrodes which are successively arranged in a direction of propagation of elastic waves.
2. Description of the Related Art
In recent years, an elastic wave filter device including a longitudinally-coupled resonator-type elastic wave filter section has been very popular for use as a filter in, e.g., an RF stage and an IF stage of a portable communication terminal because such a filter device has a small size, a light weight, and superior filter characteristics. For example, Japanese Unexamined Patent Application Publication No. 4-54011 discloses an elastic wave filter device including a 3-IDT longitudinally-coupled resonator-type elastic wave filter section.
FIG. 17 illustrates the elastic wave filter device disclosed in Japanese Unexamined Patent Application Publication No. 4-54011. As illustrated in FIG. 17, the elastic wave filter device 100 includes first and second 3-IDT longitudinally-coupled resonator-type elastic wave filter sections 101, 102 which are connected in series. An elastic wave resonator 104 is connected between a ground potential and a junction 103 at which the first 3-IDT longitudinally-coupled resonator-type elastic wave filter section 101 and the second 3-IDT longitudinally-coupled resonator-type elastic wave filter section 102 are connected to each other.
In the elastic wave filter device 100, the elastic wave resonator 104 is arranged such that the resonance frequency of the elastic wave resonator 104 is positioned within a transient band between a pass band and a stop band of each of the first and second 3-IDT longitudinally-coupled resonator-type elastic wave filter sections 101, 102. As a result, the sharpness of the filter characteristics is improved.
In the elastic wave filter device 100 including the elastic wave resonator 104 connected between the junction 103 and the ground potential, as disclosed in Japanese Unexamined Patent Application Publication No. 4-54011, the sharpness of the filter characteristics greatly depends on a Q-value of the elastic wave resonator 104. The Q-value of the elastic wave resonator 104 must be increased in order to realize higher sharpness of the filter characteristics. However, because the Q-value of the elastic wave resonator 104 is substantially determined by a piezoelectric substrate and an electrode material, the amount by which the Q-value of the elastic wave resonator 104 can be increased is limited. Thus, it is difficult to obtain a high level of sharpness of the filter characteristics, which has been in greater demand recently, merely by providing the elastic wave resonator 104.